In recent years, a high density and a high integration of semiconductor devices such as an IC, LSI, etc. are being advanced in keeping with a trend of reducing size of electronic equipments. Also, there is a tendency of narrowing spaces between electrodes and an increase in number of input and output electrodes, in a viewpoint of mounting the semiconductor devices. Furthermore, there is a demand for a reduction of thickness, as can be seen on electronic calculators, notebook type personal computers and portable telephones.
A wireless bonding method such as a flip chip method, a tape automated bonding (TAB) method, etc. are suitable for these requirements with respect to reliability concerning gang bonding and accuracy of positional alignment, reduction in thickness, high density mounting, etc. of semiconductor devices. Thus, numerous studies and developments are being made with the anticipation that it will be a great mainstay in the mounting technique of the semiconductor devices in the future.
In the wireless bonding method, a metallic protrusion called a salient electrode or a bump is usually formed on an aluminum electrode of a semiconductor device.
As means of forming the salient electrode on an aluminum electrode of a semiconductor device, many methods have been suggested, and some of them have been put to practical use. Among them, a way of forming bumps with an electroless plating method is being studied in anticipation of the low cost. One example is a palladium nucleation method which has been disclosed by Japanese Patent Laid-open Publication No. S63-305532, Japanese Patent Laid-open Publication No. S64-81344 and Japanese Patent Laid-open Publication No. H09-69524.
This method is described by referring to FIG. 10A through FIG. 10C. A surface of an aluminum electrode 3 of a semiconductor device 2 composed of a silicon substrate 1 is cleaned after being pretreated by diluted solution of nitric acid (or phosphoric acid), as shown in FIG. 10A. A numeral 4 represents a passivation film. Successively, an exposed surface of the aluminum electrode 3 is adhered with deposit 5 of palladium, as shown in FIG. 10B, by immersing the semiconductor device 2 for 30 to 60 seconds in palladium active agent consisting of 1 gram of palladium chloride, 100 c.c. of hydrochloric acid and 9.54 liters of water.
Then, as shown in FIG. 10C, the semiconductor device 2 is cleaned without removing the palladium 5 deposited on the surface of the aluminum electrode 3 of the semiconductor device. The semiconductor device 2 is then applied with electroless plating for approximately one hour by immersing it in electroless nickel plating solution having a pH of 4 to 6 and a temperature of 80 to 90.degree. C., so as to form a nickel bump 6 in a thickness of 20 .mu.m around it, including an exposed surface of the aluminum electrode 3. After that, the semiconductor device 2 is applied with an electroless gold plating, after it is rinsed, by immersing it in electroless gold plating solution in order to form a golden layer 7 of 1 .mu.m on a surface of the nickel bump 6.
However, since a catalytic treatment by the palladium active agent in the above example of the prior art has a difficulty in getting a selective plating reaction, as it deposits palladium on a surface of the semiconductor device and on a nonconductive part such as a passivation film 4 other than aluminum as being a metal, resulting in a possibility of initiating a plating reaction with the deposited palladium as a nucleus when nickel plating is being made, there are harmful effects such as deterioration of electrical insulation over the surface of a nonconductive body, an incident of short-circuiting due to a plating film bridging between the electrodes if a space between the electrodes is narrow, etc. Also, in case of an electroless nickel plating method by zincate treatment, which is usually used as a means for plating aluminum metal sheet for purposes other than semiconductors, it is not sufficient for use as an electrode for connection between the semiconductor device and the circuit substrate when the nickel plating is applied on the surface of aluminum utilized as an electrode of the semiconductor device, because roughness over a surface of the plated film is very large and an adhesion strength is weak if the aluminum used for the aluminum electrode is an alloy of, for instance, A1 with 1% Si, etc. Besides, it was observed on a semiconductor device applied with the electroless nickel plating that there are some electrodes (specific electrodes: GND, Vcc, etc. for example) on which no metal of any kind has adhered. The same tendency was seen on electrodes of many kinds of semiconductor devices, and it was a phenomenon that the plating does not adhere to electrodes of the same location.
On the other hand, there are instances where a requirement for an accuracy of height of salient electrodes is quite strict, depending on a method of mounting in case of a flip chip mounting or a TAB mounting, so that a method referred to as leveling is adopted in which the salient electrodes are compressed by putting a pressure upon them in order to make all of uniform height. Because of a problem with these kind of salient electrodes of nickel, however, that they are not compressed due to the hardness of nickel, a film of gold plating having a lower hardness is desired.
A study has been made for gold plating solution of an oxidation-reduction reacting type in order to solve the above problem. However, in the study, it was not possible to form gold into the salient electrodes with the electroless plating method, since electroless gold plating solution of the prior art, which uses potassium boron hydride or dimethyl-amine borane (DMAB), etc. as a reducing agent and gold cyanide, e.g., potassium dicyanoaurate (I), etc. as a metal base, is of high alkalinity (pH of 13 to 13.6) and the solution temperature is comparatively high as 60 to 80.degree. C., that it attacks a passivation film of the semiconductor, and there is not available a resist material of gold plating-proof. This gold plating solution also has a problem for working environments and for waste fluid disposal, because it contains cyanide.